One version of a differential pressure sensor includes spaced diaphragms and a resonator structure in a cavity between the diaphragms. The diaphragms move together in response to a differential pressure causing the mass structure of the resonator to move resulting in a strain which can be measured and correlated with a force measurement such as pressure.
Traditional differential pressure sensors are designed to determine the differential pressure between the two sides of the sensor. By way of example, traditional differential pressure sensors detect the differential pressure between two regions of interest by evaluating the net effect of the pressure forces of the two regions on a component or components of the sensor. When employed in harsh industrial environments, traditional pressure sensors often require a more robust construction. For example, if a differential pressure sensor is exposed to relatively high-pressure and/or high-temperature environments, the exposed components of the pressure sensor benefit from a construction robust enough to accommodate these conditions.
The features and attributes that facilitate operation in such high pressure (i.e., harsh) environments, however, can negatively impact the resolution of the sensor. Some traditional differential pressure sensors that are robust enough to withstand high-pressure environments, for example, cannot detect the pressure differential between the two regions of interest in orders of magnitude less than the pressure difference in the environment. For example, a resonating differential pressure sensor robust enough to withstand pressures of 5000 pounds per square inch (psi), and beyond, generally does not have sufficient resolutional capabilities to detect a pressure differential of +/−10 psi, for instance. This is because traditional resonating pressure sensors contain a vacuum within the closed enclosure between the diaphragms of the pressure sensor and therefore, with high pressures acting on the each of the diaphragm, the diaphragms may tend to bulge inside.
Thus, there is a need for a pressure sensing system and method that can provide differential pressure sensing capabilities with high resolution, while withstanding high line-pressures.
In another example, high line pressure differential pressure sensors are used to measure a small differences between two high pressures. A typical application is to measure flow in an oil pipeline. A calibrated obstruction is placed in the pipeline and the pressure difference between the two sides of the obstruction is a function of the flow rate. Typically the differential pressure is less than 10 psi while the pressure in the pipeline is 3000 psi.
In certain fault conditions, the full line pressure is applied to one side of the diaphragm with ambient pressure applied to the other. Without overpressure protection, the sensor would be destroyed. Many sensors on the market have some sort of overpressure protection mechanism built in.
Some pressure sensors of this type use a silicon pressure sensing element which can be provided with stops. A known example has a boss on the diaphragm which is a monolithic part of the silicon. There is a small gap between the boss and a substrate.
This design, however, may not provide a sufficient degree of protection for a high line pressure sensing application because the flexible region of the diaphragm is unsupported. This region has to be sufficiently flexible to sense the differential pressure which necessarily means that it is too flexible to stand the line pressure. Also, the boss stop provides protection in only one direction.
The deflection of a silicon pressure diaphragm is typically sensed by diffused strain gauges. The amount of strain needed to obtain a satisfactory signal takes the strain in the material as far towards its breaking strain consistent with an adequate margin of safety.
A stop for a flat diaphragm would need to act over the whole area and conform to the shape of the surface of the diaphragm. The movement of the diaphragm is very small. Therefore, it would be difficult to fabricate a conformal stop with sufficient accuracy.
When a diaphragm is limited by a conformal stop at high pressure, the forces on the surface are high and there is a risk that some damage of deformation may result. Such damage on the flexible part of the diaphragm carries the risk that the elastic behavior is changed and, as a result, the calibration of the instrument is altered. This is highly undesirable. It is a normal practice to replace instruments that have been subjected to line pressure in order to guarantee a know calibration.